1. Field of the Invention
The present invention relates to a gas insulated switchgear wherein mutually detachable contacts are arranged in a sealed container filled with an insulating gas, and more particularly relates to a gas insulated switchgear having excellent interrupting performance while using an insulating gas having a global warming potential lower than that of SF6 gas.
2. Description of the Related Art
Depending on their intended use and required functionality, a gas insulated switchgear having a current interruption function include, for instance, load switches, disconnecting switches, circuit breakers and the like. In many of these devices, a pair of contacts is arranged within an insulating gas such as SF6 gas or the like, so that during conduction electricity is conducted by maintaining the two contacts in a contact state, while during current interruption the contacts open, an arc discharge occurs thereupon in the gas, and then current is interrupted by extinguishing the arc.
Conventional technology will be explained herein using as an example a puffer-type gas blast circuit breaker widely employed in protection switchgear in high-voltage transmission systems operating at 72 kV or more. FIG. 9 is an example of a schematic cross-sectional diagram of such a gas blast circuit breaker, depicted during the interrupting operation. The various components in FIG. 9 may be assumed to be coaxial cylinders.
As illustrated in FIG. 9, an insulating gas 2 is sealed in a sealed container 1 that comprises a grounded metal, a porcelain tube and the like. Inside the sealed container 1 are arranged a fixed contact section 21 and a movable contact section 22 opposite each other, with a fixed arc contact 7a and a movable arc contact 7b provided respectively on the fixed contact section 21 and the movable contact section 22. The support structure for these arc contacts inside the sealed container 1 is omitted in the figure.
During normal operation, the arc contacts 7a and 7b are in a contact conductive state, while during an interrupting operation the contacts 7a and 7b open by moving relative to each other, whereupon an arc 8 is formed in the space between the two contacts. On the side of the movable contact section 22 there is further arranged a gas flow generating means for blowing the insulating gas 2, as an arc extinguishing gas, onto the arc 8.
As the gas flow generating means there are provided herein a piston 3, a cylinder 4, a puffer chamber 5, and an insulating nozzle 6. On the side of the fixed contact section 21 there is provided a metallic discharge tube 9 through which fixed side hot gas flow 11a can pass. On the side of the movable contact section 22 there is provided a hollow rod 12, joined to the movable arc contact 7b, through which movable side hot gas flow 11b can pass.
In the interrupting process of a gas blast circuit breaker having the above constitution, when the movable contact section 22 operates in the left direction of the figure, the fixed piston 3 compresses the puffer chamber 5, which is the inner space of the piston 4, thereby raising the pressure in that section. The insulating gas 2 in the puffer chamber 5 becomes a high-pressure gas flow that is led to the insulating nozzle 6, and is vigorously blown onto the arc 8 formed between the arc contacts 7a and 7b. As a result, the electroconductive arc 8 formed between the contacts 7a and 7b is extinguished and current is shut off.
As is known, the higher the pressure inside the puffer chamber 5, the more strongly the insulating gas 2 is ordinarily blown towards the arc 8, and thus the current interrupting performance that is obtained becomes higher. By being blown onto the high-temperature arc 8, the insulating gas 2 becomes a high-temperature gas that, in the form of the fixed side hot gas flow 11a and the movable side hot gas flow 11b, flows away from the space between the two arc contacts, dispersing eventually into the interior of the sealed container 1. Although not shown in the figure, sliding portions such as the gap between the piston and the cylinder are often greased in order to reduce friction.
The above is a typical constitution of a puffer-type gas blast circuit breaker that is an example of the gas insulated switchgear. Schemes have been proposed in recent years for achieving a higher current interrupting performance by, in addition to the mechanical compression by the piston 3, actively exploiting the thermal energy of the arc 8.
During the early stages of the interrupting operation, for instance, the movable side hot gas flow 11b may be brought into the puffer chamber 5 via holes provided in the hollow rod 12 (Japanese Examined Patent Application Publication No. H07-109744). Alternatively, the puffer chamber 5 may be split in two along the axial direction, so that a high blowing pressure onto the arc 8 may be obtained, in particular during large-current interruption, by limiting the volume of the puffer chamber 5 in the vicinity of the arc 8. Also, a check valve provided at the split section of the puffer chamber 5 prevents a direct high-pressure action of the piston 3, thereby reducing the force with which the movable contact section 22 is driven (Japanese Examined Patent Application Publication No. H07-097466).
SF6 gas, or air, is often used as the insulating gas 2 in the gas insulated switchgear that has become widespread in recent years. SF6 gas has excellent characteristics as regards arc extinction (arc extinguishing performance) and electrical insulation performance, and is widely used, in particular, in a high-voltage gas insulated switchgear. Air is used, in particular, in a small-sized gas insulated switchgear owing to its low cost and its excellent safety and environmental compatibility.
Although SF6 gas is extremely useful in a high-voltage gas insulated switchgear, its use is expected to fall on account of its known significant global warming effect. The magnitude of global warming effect is expressed ordinarily as a global warming potential relative to a global warming potential, i.e. a relative value with respect to a global warming potential of 1 for CO2. As is known, SF6 gas has a global warming potential reaching up to 23900. Moreover, although its safety and environmental compatibility is excellent, air is far poorer than SF6 gas in terms of arc extinguishing performance and electrical insulation performance, and hence its large-scale use in a high-voltage gas insulated switchgear is fraught with difficulties.
In this context, CO2 gas has been proposed as an arc extinguishing gas for the gas insulated switchgear (Uchii, Kawano, Nakamoto and Mizoguchi, “Validation of thermal interrupting performance of CO2 gas as an arc extinguishing medium on the basis of fundamental properties of CO2 gas and a full-scale model circuit breaker” IEEJ Transactions on Power and Energy, vol. 124, 3, pp. 469-475, 2004). The global warming effect of CO2 gas is extremely small relative to that of SF6 gas, of 1/23900, and hence using CO2 gas alone, or a mixed gas with CO2 gas as a main constituent thereof (main constituent being defined herein as the constituent present in the gas at a proportion of 50% or more) should allow significantly restraining the impact of the gas on global warming by replacing SF6 gas in the gas insulated switchgear.
Although the arc extinguishing performance and electric insulation performance of CO2 gas are inferior to those of SF6 gas, the arc extinguishing performance of CO2 is far superior to that of air, while its insulation performance is similar to that of air, or greater. Thus, using CO2 gas by itself, or using a mixed gas having CO2 gas as the main constituent, in lieu of SF6 gas or air, allows providing a gas insulated switchgear having good characteristics overall, and environmentally superior in terms of restraining global warming impact.
When CO2 gas is used in a puffer-type gas insulated switchgear such as the one illustrated in FIG. 9, the scheme proposed in Japanese Examined Patent Application Publication Nos. H07-109744 and H07-097466, of exploiting the thermal energy of the arc 8, is noticeably effective in enhancing the intrinsic effect of CO2 gas (Uchii, Kawano, Nakamoto and Mizoguchi, “Validation of thermal interrupting performance of CO2 gas as an arc extinguishing medium on the basis of fundamental properties of CO2 gas and full-scale model circuit breaker” IEEJ Transactions on Power and Energy, vol. 124, 3, pp. 469-475, 2004).
Apart from CO2 gas, other gases have been proposed, for the same reasons expounded above, as insulating gases for use in the gas insulated switchgear. These gases include, for instance, a perflurocarbon such as CF4 gas or the like, a hydrofluorocarbon such as CH2F2 gas or the like (“Environmental burden of SF6 and gas insulation mixtures /alternatives to SF6”, IEEJ technical communications, 841, 2001), and CF3I gas (Japanese Patent Application Laid-open No. 2000-164040). These gases have less impact on global warming than SF6 gas, and have relatively high arc extinguishing performance and insulation performance, and hence are effective in reducing the environmental burden of the gas insulated switchgear.